In recent years, different types of small-sized and highly portable electronic devices, such as smartphones or tablet PCs, have been developed along with the advancement of electronic technology and communication technology. Such devices are usually integrated with many functions in order to improve their market competitiveness. Besides appearance, display quality, and processing speed, sound effect is also a major factor considered by the customers in purchasing such products.
Due to limited space, smartphones are usually equipped with only one speaker for audio playback, which results in monotonous sound effect since the sound is produced from only one speaker. Based on the knowledge that a stereo effect requires at least two speakers, smartphone manufacturers re-design the device structure to further place a speaker on the front plane of the device but at an opposite side to an earpiece (i.e. a part of a device that is placed in accordance with the ear for listening to something) of the device. This speaker is accompanied with the earpiece to respectively produce a left channel sound and a right channel sound of an audio played by the device so as to provide a stereo sound effect. However, by using the earpiece as the speaker of one sound channel, when the user just ends a call or the user's ear is near the earpiece due to any purpose, an unexpected loud sound produced by the earpiece due to another call, a notification, or an alarm may shock the user since the user's ear is near the earpiece.